1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing composite electrical insulators made of polymeric material.
2. Description of Related Art
There is known a composite electrical insulator which includes an elongate and solid or hollow core rod of fiber-reinforced plastics (FRP), a sheath covering the core rod over substantially the entire length thereof, and a plurality of sheds projecting radially outwards from the sheath and spaced from each other in a longitudinal direction of the core rod. The sheath and the sheds are integrally formed on the core rod by injection or transfer molding of appropriate polymeric material having an electrically insulating property, such as silicone rubber, ethylene propylene copolymer (EPM), ethylene propylene diene copolymer (EDPM), polyurethane, etc. Such a composite insulator has been actually put into practical applications, particularly in any use environment which can draw out various functional advantages of the composite insulator.
There have been various proposals regarding the technology for manufacturing composite insulators. For example, JP-A-54-65392 discloses a method for manufacturing composite insulators wherein the sheds are mounted onto the sheath on the core rod in their radially expanded state, and carried on the sheath by a clamping force which arises from a radial shrinkage and adhered thereto by an adhesive agent. This method suffers from a problem that it is difficult to improve the manufacturing productivity and assembling accuracy, particularly when the sheds are mounted onto the sheath by a manual operation. Furthermore, JP-A-61-181015 discloses another method wherein the shed is caused to radially expand in a room temperature condition, is cooled and mounted onto the sheath on the core rod in its expanded state, and is heated to the room temperature such that the shed undergoes a shrinkage so as to be tightly connected to the sheath. This method also suffers from a problem that it is difficult to improve the manufacturing productivity and assembling accuracy. Moreover, precise and strict temperature control is required for the sheds.